a. Field of the Invention
The present invention relates to an electric terminal for an electronic device and, more particularly, to the structure of an electric terminal for use in an electronic device and having a higher connection reliability. The present invention also relates to a method for forming such an electric terminal.
b. Description of the Related Art
The reliability of an electronic device often depends on the connection reliability of the electric terminals used in the electronic devices. FIGS. 1A to 1D show conventional electric terminals used in semiconductor devices, which are to be mounted on respective printed circuit boards (mounting board) by using a flip-chip bonding technique. FIGS. 2A to 2D corresponding to FIGS. 1A to 1D, respectively, show the electric terminals after the heat cycle tests thereof.
FIG. 1A shows the basic structure of the flip-chip bonding terminal called C4 type, wherein an external electrode implemented by a solder ball 32 formed on an electronic device 31 is mounted on a pad 34 of a printed circuit board 33, and is melted thereon for bonding.
In general, after the melting and subsequent cooling steps of the mounting process, an under-filling process is conducted wherein the space between the electronic device 31 and the printed circuit board 33 is filled with resin, which is then subjected to a curing step. The under-filling process improves the reliability of the bonding structure during the subsequent heat cycle test by diversifying the thermal stress applied onto the bonding structure during the heat cycle test. The heat cycle test is likely to cause a crack 38 in the solder ball 32, such as shown in FIG. 1B.
The is a problem in the basic structure of the flip-chip bonding structure of FIG. 1A that the under-filling step increases the fabrication steps and thus raises the cost of the electronic device. In addition, the smaller pitch of the arrangement for the external terminals of the up-to-date electronic device due to the higher integration of the semiconductor devices renders the under-filling step itself difficult.
FIGS. 1B to 1D are improved conventional flip-chip bonding structures devised or proposed in view of the above problems in the basic structure shown in FIG. 1A.
In FIG. 1B, an interposer substrate or sheet 35 is interposed between the electronic device 31 and the solder ball 32 for alleviating or absorbing the stress. The electric connection between the electronic device 31 and the solder ball 32 is implemented by an interconnecting wire or an inner lead of a tape automated bonding (TAB) structure.
It is difficult to use the interposer substrate 35, however, for an electronic device having higher-density terminals because of the difficulty in arranging a large number of interconnecting wires with a higher density. In addition, the thermal expansion of material used for the interposer sheet may damage the bonding structure to cause a crack 38 in the solder ball, as shown in FIG. 2B.
In FIG. 1C, a conductive post or pole 36 such as made of Cu is formed on the electronic device 31, and the solder ball 32 is disposed on the top of the conductive post 36. The conductive post 36 should have a diameter sufficient for supporting the solder ball 32 and a height sufficient for absorbing the stress.
It is difficult, however, to design the diameter and the height of the conductive post 36 because a trade-off resides between the handling or testing of the electronic device and the reliability for the bonding structure of the electronic device. More specifically, the handling or testing step requires a larger diameter and a smaller height of the conductive post 36 to have a sufficient mechanical strength, whereas a higher reliability in the bonding structure requires a smaller diameter and a large height of the conductive post 36 for absorbing the thermal stress to avoid a crack 38 in the solder ball 32 (FIG. 2C).
In FIG. 1D, a conductive spring or wire 37 is used instead of the solder ball 32 for achieving a higher density of the external terminals. The surface of the wire 37 is coated with a thick plating film for achieving a sufficient mechanical strength and resilience.
The wires 37, however, have a lower allowance for a misalignment in arrangement of the terminals and for the uniformity in the height of the terminals, i.e., for the coplanarity in arrangement of the terminals, compared to the solder balls which allow a relatively large deviation in the arrangement and the height. This lowers the product yield of the electronic devices having such wires 37 irrespective of employing a BGA structure for the electric terminals.
In view of the above problems in the conventional techniques, it is an object of the present invention to provide an electric terminal for use in an electronic device.
It is another object of the present invention to provide a method for forming such an electric terminal on an electronic device.
The present invention provides an electric terminal for an electronic device including: an external electrode; a lead member disposed on an internal electrode of the electronic device, at least a portion of the lead member being a conductor connecting the external electrode and the internal electrode; and a support member disposed on the electronic device in the vicinity of the lead member for supporting the external electrode at least upon application of an external thrust force which deforms the lead member.
The present invention also provides a method for forming an electric terminal on an electronic device including the steps of: forming a lead member on an internal electrode of the electronic device; forming an external electrode on the lead member; and forming a support member on the electronic device in a vicinity of the lead member, the support member being in contact with the external electrode at least upon application of an external force which deforms the lead member.
In accordance with the electric terminal of the present invention and the electric terminal manufactured by the method of the present invention, by separating the lead member having an electric connection function of the electric terminal from the support member having a mechanical supporting function of the electric terminal, a reliable electric connection can be obtained after a heat cycle test for the electronic device.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.